The goals of the present project are to investigate the neural mechanisms of selective attention and attention orienting in patients with either focal cortical damage or complete cerebral commissurotomy, as well as in normal control subjects. In a series of experiments this project will investigate the brain systems that mediate selective attention to spatial locations, to different classes of stimulus features such as color, movement and spatial frequency, and to conjunctions of features (objects). Evidence from animal and human studies indicate that the parietal cortex is principally involved in aspects of spatial cognition while the inferotemporal projection system is involved in the representation of stimulus features and objects. We hypothesize that, in part, attentional selection modulates activity in the cortical areas that code a particular visual feature or dimension, and we therefore predict that spatial aspects of attention require the integrity of the parietal cortex while attention to features and objects is a function of the occipito-inferotemporal projection pathway. Moreover, it has been hypothesized that differential specializations of the two hemispheres for spatial versus feature- and object-based perception exist such that the left hemisphere is biased towards object representation while the right hemisphere is dominant for spatial processing. We will utilize behavioral measures of attention (response time and accuracy) and electrophysiological measures (i.e., ERPs) of parietal and infero- temporal visual processing in studies of patients with parietal-only versus temporo-parietal cortex lesions of both the left and right hemispheres to investigate the role of these brain areas in spatial, feature-based, and object-based attention. Studies in commissurotomy patients will also investigate the hypothesized hemispheric asymmetries in spatial and object-based selective attention, as well as hemispheric specializations in the processing of global versus local levels of stimuli. Investigations of subcortical and cortical interactions during attention in the commissurotomy patients will continue our work in this area. Previous results have demonstrated that under many conditions the cortically separated hemispheres of commissurotomy patients can act independently during visual search for target items in large arrays of distractors, while continuing to demonstrate interhemispheric interactions in perceptual-motor processing. We will attempt to identify the critical conditions that determine whether or not the two cerebral hemispheres can perform independently during attentional processing. These studies will help to identify cortical systems and hemispheric specializations in attentional processing as well as the contributions of subcortical systems to the integration of attentional processes in the human brain.